Somehow I can't see Man Utd throw away the PL and CL. They'll at least get one bit of silverware (in terms of either PL or CL) whereas I think Chelsea are burning their players trying for both CL and PL. I don't think they have the depth/strength of team squad play as Man Utd, and you could see that during the game between the two (referring to the Drogba/Ballack free kick and the aftermath).
Arsenal could still win the PL...even if it is an outside chance, it could simply be a case both Man Utd and Chelsea concentrate on the CL and let the last remaining games slip by fielding tired or second/third string players.